Long term storage properties of potato represents a major determinant of tuber quality. Dormancy periods (the time period after harvesting and before sprouting) are crucial to maintaining quality potatoes. Commercially, potatoes may be held for long periods before processing (up to 10 months or longer), and at temperatures typically between 2.degree.-10.degree. C. Cold storage (2.degree.-6.degree. C.) versus storage at 7.degree.-12.degree. C. provides the best long term conditions by reducing respiration, water loss, microbial infection, and the need for chemical sprout inhibitors (Burton, 1989). However, low temperatures lead to cold-induced sweetening, and the resultant high sugar levels contribute to an unacceptable brown color in the fried product (Coffin et al., 1987, Weaver et al., 1978). The sugars that accumulate are predominantly glucose, fructose, and sucrose, and it is mainly the reducing sugars (primarily glucose and fructose) which react with free amino groups upon heating during the various cooking processes, including frying, via the Maillard reaction, and result in the formation of brown pigment (Burton, 1989, Shallenberger et al., 1959). Sucrose, on the other hand, produces a black coloration on frying due to its susceptibility to undergo carmelization as well as charring. Levels of reducing sugars above 0.2% fresh weight are sufficient to cause brown pigment formation and thus merit rejection for certain types of processing. A potato processor can reduce the levels of sugars by a costly and time consuming blanching process if the levels of sugars are not significantly higher than the 0.2% limit. Potatoes can be reconditioned at higher temperatures (18.degree. C.) to lower sugar content, but often sugar levels will not sufficiently decrease before the onset of sprouting at these temperatures, requiring the use of chemical sprout inhibitors (Ryall and Lipton, 1979, Hardenburg et al., 1986). However, reconditioning increases the storage facility requirements and consequently affects the final cost of the product. Furthermore, it has been shown that reconditioning is not effective after longer storage periods (Coffin et al., 1987). Given the negative environmental and health perceptions associated with excessive chemical use, and the fact that current sprout inhibitors may soon be banned, a need exists for potato varieties which can withstand long term cold storage without the use of chemicals, without accumulation of reducing sugars, and with greater retention of starch levels.
After longer storage periods, sprouting of potato tubers becomes a problem. Excess sprouting reduces the market value and can cause increased levels of alkaloids in the tuber.
Through the process of genetic engineering potato tubers which contain significantly higher levels of starch have been obtained. See WO 91/19806 (Kishore), also U.S. Ser. No. 07/709,663, filed Jun. 7, 1991, now abandoned, hereby incorporated by reference. In these tubers a gene is expressed which encodes ADPglucose pyrophosphorylase (ADPGPP), which catalyzes a key step in starch and glycogen biosynthesis. The preferred gene is from E. coli and the resulting enzyme is a poorly regulated, highly active variant. When a mutant of this gene, glgC16, is expressed in a tuber-specific manner, for example from a class I patatin promoter, starch levels are higher than those of nontransgenic control tubers at the time of harvest.
Carbohydrate metabolism is a complex process in plant cells. Manipulation of a number of different enzymatic processes potentially may effect the accumulation of reducing sugars during cold storage. For example, sugars may be used to resynthesize starch, and thus effect reduction in the pool of free sugar. Other methods may also serve to enhance the cold storage properties of potato through reduction of sugar content, including the inhibition of starch hydrolysis, removal of sugars through glycolysis, or conversion of sugars into other forths which would not participate in the Maillard reaction. The challenge in these methods would be to identify an activity with which to effect the desired result, achieve function at low temperatures, and still retain the product qualities desired by potato growers, processors, and consumers.
It has been suggested that phosphofructokinase (PFK) plays an important role in the cold-induced sweetening process (Kruger and Hammond, 1988, ap Rees et al., 1988, Dixon et al., 1981, Claassen et al., 1991). ap Rees et al. (1988) suggested that cold treatment had a disproportionate effect on different pathways in carbohydrate metabolism in that glycolysis was more severely reduced due to the cold-lability of PFK. The reduction in PFK activity would then lead to an increased availability of hexose-phosphates for sucrose production. Additional support for this view comes from the observation of a new breeding clone of potato which contains a PFK which is not cold labile and that does not accumulate significant amounts of sugar in the cold.
It was recently disclosed in European Patent Application 0 438 904 that increasing PFK activity reduces sugar accumulation during storage by removing hexoses through glycolysis and further metabolism. A PFK enzyme from E. coli was expressed in potato tubers and the report claimed to increase PFK activity and to reduce sucrose content in tubers assayed at harvest. However, it has been shown that pyrophosphate:Fructose 6-phosphate phosphotransferase (PFP) remains active at low temperatures (Claassen et al., 1991). PFP activity can supply fructose 6-phosphate for glycolysis just as PFK can since the two enzymes catalyze the same reaction. Therefore the efficacy of this approach in improving the cold storage quality of potato tubers remains in doubt. Furthermore, the removal of sugars through glycolysis and further metabolism would not be a preferred method of enhancing storage properties of potato tubers because of the resultant loss of valuable dry matter content through respiration. Resynthesis of the sugars into starch or slowing the breakdown of starch would be preferred because dry matter would be retained.
It is an object of this invention to provide a method for reducing the level of sugars within potato tubers and to provide improved quality of stored potatoes. It is a further object of this invention to provide potatoes having an improved rate and degree of reconditioning after storage at reduced temperatures. It is a still further object of this invention to provide a method of extending dormancy of potatoes stored at ambient temperatures or at reduced temperatures.